Fabrication of circuit modules with a transmission line

ABSTRACT

Interconnections are made through a planar circuit by a monolithic short-circuited transmission path which extends from a circuit portion of the planar circuit to the opposite side. The opposite side is ground sufficiently to remove the short-circuiting plate, thereby separating the previously monolithic conductors, and exposing ends of the separated conductors of the transmission path. Connection is made between the exposed conductors of the transmission path and the registered contacts of a second planar circuit by means of electrically conductive, compliant fuzz buttons. The transmission path may be a coaxial path useful for RF.

FIELD OF THE INVENTION

This invention relates to RF (including microwave) interconnectionsamong layers of assemblies of multiple integrated circuits, and moreparticularly to compliant interconnection arrangements which may besandwiched between adjacent circuits.

BACKGROUND OF THE INVENTION

Active antenna arrays are expected to provide performance improvementsand reduce operating costs of communications systems. An active antennaarray includes an array of antenna elements. In this context, theantenna element may be viewed as being a transducer which convertsbetween free-space electromagnetic radiation and guided waves. In anactive antenna array, each antenna element, or a subgroup of antennaelements, is associated with an active module. The active module may bea low-noise receiver for low-noise amplification of the signal receivedby its associated antenna element(s), or it may be a power amplifier foramplifying the signal to be transmitted by the associated antennaelement(s). Many active antenna arrays use transmit-receive (T/R)modules which perform both functions in relation to their associatedantenna elements. The active modules, in addition to providingamplification, ordinarily also provide amplitude and phase control ofthe signals traversing the module, in order to point the beam(s) of theantenna in the desired direction. In some arrangements, the activemodule also includes filters, circulators, andor other functions.

A major cost driver in active antenna arrays is the active transmit orreceive, or T/R module. It is desirable to use monolithic microwaveintegrated circuits (MMIC) to reduce cost and to enhance repeatabilityfrom element to element of the array. Some prior-art arrangements useceramic-substrate high-density-interconnect (HDI) substrate for theMMICs, with the substrate mounted to a ceramic, metal, or metal-matrixcomposite base for carrying away heat. These technologies are effective,but the substrates may be too expensive for some applications.

FIG. 1 illustrates a cross-section of an epoxy-encapsulated HDI module10 in which a monolithic microwave integrated circuit (MMIC) 14 ismounted by way of a eutectic solder junction 16 onto the top of aheat-transferring metal deep-reach shim 18. The illustrated MMIC 14,solder 16, and shim 18 are encapsulated, together with other like MMIC,solder and shim assemblies (not illustrated) within a plasticencapsulant or body 12, the material of which may be, for example, epoxyresin. The resulting encapsulated part, which may be termed"HDI-connected chips" inherently has, or the BP ΔN lower surfaces areground and polished to generate, a flat lower surface 12_(ls) BP ΔN. Theflat lower surface 12_(ls), and the exposed lower surface 18_(ls), ofthe shim, are coated with a layer 20 of electrically and thermallyconductive material, such as copper or gold. As so far described, themodule 10 of FIG. 1 has a plurality of individual MMIC mounted orencapsulated within the plastic body 12, but no connections are providedbetween the individual MMICs or between any one MMIC and the outsideworld. Heat which might be generated by the MMIC, were it operational,would flow preferentially through the solder junction 16 and the shim 18to the conductive layer 20.

In FIG. 1, the upper surface of MMIC 14 has two representativeelectrically conductive connections or electrodes 14₁ and 14₂.Connections are made between electrodes 14₁ and 14₂ and thecorresponding electrodes (not illustrated) of others of the MMICs (notillustrated) encapsulated within body 12 by means of HDI technology,including flexible layers of KAPTON on which traces or patterns ofconductive paths, some of which are illustrated as 32₁ and 32₂, havebeen placed, and in which the various layers are interconnected by meansof conductive vias. In FIG. 1, KAPTON layers 24, 26, and 30 are providedwith paths defined by traces or patterns of conductors. The layersillustrated as 24 and 26 are bonded together to form a multilayer,double-sided structure, with conductive paths on its upper and lowersurfaces, and additional conductive paths lying between layers 24 and26. Double-sided layer 24/26 is mounted on upper surface 12_(us) of body12 by a layer 22 of adhesive. A further layer 30 of KAPTON, with its ownpattern of electrically conductive traces 32₂, is held to the uppersurface of double-sided layer 24/26 by means of an adhesive layer 28.The uppermost layer of electrically conductive traces may includeprinted antenna elements in one embodiment of the invention. Asmentioned above, electrical connections are made between the conductivetraces of the various layers, and between the traces and appropriateones of the MMIC contacts 14₁ and 14₂, by through vias, some of whichare illustrated as 36. The items designated MT0, MT1, MT2, and MT3 atthe left of FIG. 1 are designations of various ones of the flexiblesheets carrying the various conductive traces. Those skilled in the artwill recognize this structure as being an HDI interconnectionarrangement, which is described in U.S. Pat. No. 5,552,633, issued Sep.3, 1996 in the name of Sharma.

As illustrated in FIG. 1, at least one radio-frequency (RF) groundconductor layer or "plane" 34 is associated with lower layer 24 of thedouble-sided layer 24/26. Those skilled in the art will realize that thepresence of ground plane 34 allows ordinary "microstrip"transmission-line techniques to carry RF signals in lateral directions,parallel with upper surface 12_(us) of plastic body 12, so that RFsignals can also be transmitted from one MMIC to another in the assembly10 of FIG. 1.

Allowed U.S. patent application Ser. No. 08/815,349, in the name ofMcNulty et al., describes an arrangement by which signals can be coupledto and from an HDI circuit such as that of FIG. 1. As described in theMcNulty et al. application, the HDI KAPTON layers with their patterns ofconductive traces are lapped over an internal terminal portion of ahermetically sealed housing. Connections are made within the body of thehousing between the internal terminal portion and an externallyaccessible terminal portion.

One of the advantages of an antenna array is that it is a relativelyflat structure, by comparison with the three-dimensional curvature ofreflector-type antennas. When assemblies such as that of FIG. 1 are tobe used for the transmit-receive modules of an active array antenna, itis often desirable to keep the structure as flat as possible, so as, forexample, to make it relatively easy to conform the antenna array to theouter surface of a vehicle. FIG. 2a illustrates an HDI module such asthat described in the abovementioned McNulty patent application. In FIG.2a, representative module 210 includes a mounting base 210, to whichheat is transferred from internal chips. A plurality of mounting holesare provided, some of which are designated 298. A contoured lid 213 ishermetically sealed to a peripheral portion of base 212, to protect thechips within. A first set of electrical connection terminals, some ofwhich are designated as 222a, 224a, and 226a are illustrated as beinglocated on the near side of the base, and a similar set of connectionterminals, including terminals designated as 222b, 224b, and 226b arelocated on the remote side of the base. FIG. 2b is a perspective orisometric view, partially exploded, of an active array antenna 200. InFIG. 2b, the rear or reverse side (the non-radiating or connection side)of a flat antenna element structure 202 is shown, divided into rowsdesignated a, b, c, and d and columns 1, 2, 3, 4, and 5. Each locationof array structure 202 is identified by its row and column number, andeach such location is associated with a set of terminals, three innumber for each location. Each array location of antenna element array202 is associated with an antenna element, which is on the obverse orfront side of structure 202. Each antenna element on the obverse side ofthe antenna element structure 202 is connected to the associated set ofthree terminals on the corresponding row and column of the reverse sideof the antenna element array 202. Each antenna element of active antennaarray 200 of FIG. 2b is associated with a corresponding active antennamodule 210, only one of which is illustrated. In FIG. 2b, active antennamodule 210b3 is associated with antenna element or array element 202b3.Active module 210b3 is identical to module 210 of FIG. 2a and to all ofthe other modules (not illustrated) of FIG. 2b. Representative module210b3 has its terminals 222a, 224a, and 226a connected by means ofelectrical conductors to the set of three terminals associated witharray element 202b3 of antenna structure 202. The other set of terminalsof module 210b3, namely the set including terminals 222b, 224b, and226b, is available to connect to a source or sink of signals which areto be transmitted or received, respectively. It will be clear that theorientation of module 210b3, and of the other modules which itrepresents, will, when all present, will extend for a significantdistance behind or to the rear of the antenna element support structure202, thereby tending to make the active antenna array 200 fairly thick.Also, the presence of the many modules will make it difficult to supportthe individual modules in a manner such that heat can readily beextracted from the mounting plates (212 of FIG. 2a). Also, the presenceof many such active modules 210 will make it difficult to make theconnections between the terminal sets of the active modules and theterminal sets of the antenna elements. The problem of thickness of thestructure of FIG. 2b is exacerbated by the need for a signaldistribution arrangement, partially illustrated as 290. Distributionarrangement 290 receives signal from a source 292, and distributes someof the signal to the near connections of each of the modules, such asconnections 222b. 224b, and 226b of module 210b3.

A further problem with the structure of FIG. 2b is that the connectionsbetween the active module 210b3 and the set of terminals for arrayelement 202b3 is by way of an open transmission-line. Those skilled inthe art of RF and microwave communications know that such opentransmission-lines tend to be lossy, and in a structure such as thatillustrated in FIG. 2b, the losses will tend to result in cross-couplingof signal between the terminals of the various array elements.

A further problem with interconnecting the structure of FIG. 2b is thatof tolerance build-up between the antenna terminal sets on the reverseside of the antenna element structure 202, the terminals of the modules210, and the terminals of beamformer 290.

Improved arrangements are desired for producing flat HDI-connectedstructures which can be arrayed with other flat structures.

SUMMARY OF THE INVENTION

In one aspect, the invention lies in a short electricaltransmission-line which includes a center electrical conductor havingthe form of a circular cylinder centered about an axis. The circularcylinder of the center conductor defines an axial length between firstand second ends of the center conductor. An outer electrical conductorarrangement comprises a plurality of mutually identical electrical outerconductors, each being in the form of a circular cylinder centered aboutan axis, and each having an axial length between first and second endswhich is equal to the axial length of the center conductor. The axes ofthe outer conductors are oriented parallel with each other and with theaxis of the center conductor. The first ends of the center and outerconductors are coincident with a first plane which is orthogonal to theaxes of the center and outer conductors, and the second ends of thecenter and outer conductors are coincident with a second plane parallelwith the first plane. The outer conductors have their axes equallyspaced from each other at a first radius from the axis of the centerconductor. The short electrical transmission-line also includes a rigiddielectric disk defining a center axis and an axial length no greaterthan the axial length of the center conductor. The rigid dielectric diskalso defines a periphery spaced from the center axis by a second radiuswhich is greater than either (a) the first radius or (b) the axiallength of the center conductor. The dielectric disk surrounds andsupports the center and outer conductors on side regions thereof lyingbetween the first and second ends of the center and outer conductors,for holding the center and outer conductors in place. However, thedielectric disk does not overlie the first ends of the center and outerconductors.

In a more particular embodiment, the center conductor defines adiameter, and the outer conductors each have the same diameter. Moreparticularly, the material of the center and outer conductors comprisesat least a copper core, and the material of the dielectric disk is epoxyresin.

A method, according to an aspect of the invention, for producing a flatconnection assembly includes the step of affixing a plurality ofmicrowave integrated-circuit chips to a support, with connections of thechips adjacent to the support. A plurality of short electricaltransmission-lines are made or generated. Each of the short electricaltransmission-lines is similar to that summarized above. A plurality ofthe short transmission-lines are applied to the support, with the firstends of the conductors adjacent the support. The chips and the shorttransmission-lines are encapsulated in rigid dielectric material, tothereby produce encapsulated chips and transmission-lines. The supportis removed from the encapsulated chips and transmission-lines, tothereby expose a first side of the encapsulated chips andtransmission-lines, and at least the connections of the chips and thefirst ends of the center and outer conductors of the shorttransmission-lines. At least one layer of flexible dielectric sheetcarrying a plurality of electrically conductive traces is applied to thefirst side of the encapsulated chips and transmission-lines. Theflexible dielectric sheet interconnects, by way of some of the tracesand by through vias, at least one of the connections of at least one ofthe chips with the first end of the center conductor of one of thetransmission-lines, and at least one other of the connections of the oneof the chips to the first ends of all of the outer conductors of the oneof the transmission-lines, to thereby produce a first-side-connectedencapsulated arrangement. So much material is removed from that side ofthe first-side-connected encapsulated arrangement which is remote fromthe first side as will expose second ends of the center and outerconductors of the transmission-lines, to thereby produce a first planararrangement having exposed second ends of the center and outerconductors of the transmission-lines. A planar conductor arrangement isapplied over the first planar arrangement, and adjacent that side of thefirst planar arrangement which has the exposed second ends of the centerand outer conductors. The planar conductor arrangement includes aplurality of individual electrical connections which, when the planarconductor arrangement is registered with the first planar arrangement,are registered with the center and outer conductors of thetransmission-lines. The planar conductor arrangement is registered withthe first planar arrangement, and electrical connections are madebetween the first ends of the center and outer conductors of thetransmission lines of the first planar arrangement and the individualelectrical connections of the planar conductor arrangement.

In a particular method according to an aspect of the invention, the stepof making electrical connections comprises the steps of placing acompressible floccule of electrically conductive material between thefirst ends of each of the center and outer conductors of thetransmission lines of the first planar arrangement and the registeredones of the planar conductor arrangement, and compressing thecompressible floccule of electrically conductive material between thefirst ends of the center and outer conductors of the transmission linesof the first planar arrangement and the registered ones of the planarconductor arrangement, to thereby establish the electrical connectionsand to aid in holding the compressible floccules in place. The step ofencapsulating the chips and the short transmission-lines in dielectricmaterial includes the step of encapsulating the chips and the shorttransmission-lines in the same dielectric material as that of thedielectric disk.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional view of a portion of a prior-arthigh-density interconnect arrangement by which connections are madebetween multiple integrated-circuit chips mounted on a single supportingsubstrate;

FIG. 2a is a simplified perspective or isometric view of a prior-artmodule which contains HDI-connected integrated-circuit chips, and FIG.2b illustrates how a flat or planar antenna array might use a pluralityof the modules of FIG. 2a to form an active antenna array;

FIGS. 3a and 3b are simplified plan and elevation views, respectively,of a short transmission-line, and

FIG. 3c is a cross-section of the structure of FIG. 3a taken alongsection lines 3c--3c;

FIGS. 4a, 4b, 4c, 4d, 4e, 4f, 4g, and 4h illustrate steps, in simplifiedform, in the fabrication of an RF HDI structures using a shorttransmission-line as in FIGS. 3a, 3b, and 3c to interface to anotherplanar circuit, illustrated as a beamformer or manifold;

FIG. 5 illustrates an arrangement similar to that of FIG. 4h with a coldplate interposed between the HDI-connected chips and the beamformer, andusing a rigid fuzz button holder;

FIG. 6a is a simplified plan view of a compressible or conformable shorttransmission line,

FIG. 6b is a simplified cross-section of the arrangement of FIG. 6ataken along section lines 6a--6a,

FIG. 6c is a simplified perspective or isometric view of the shorttransmission line of FIGS. 6a and 6b, with the fuzz button conductorsillustrated in phantom, and

FIG. 6d is a simplified perspective or isometric view of arepresentative fuzz button;

FIG. 7 is a simplified cross-sectional representation of an assemblageincluding a cold plate, in which a compressible fuzz button holder isused;

FIG. 8 is a simplified perspective or isometric view, exploded to revealcertain details, of the assemblage of FIG. 7;

FIG. 9a is a simplified perspective or isometric view of ashort-circuited transmission line according to an aspect of theinvention,

FIG. 9b is a side or elevation view of the transmission line of FIG. 9a,

FIG. 9c illustrates the arrangement of FIG. 9a in encapsulated form, and

FIG. 9d is a side elevation of the encapsulated structure of FIG. 9c;

FIG. 10a illustrates the result of certain fabrication stepscorresponding to the steps of FIGS. 4a, 4b, 4c, and 4d applied to theshort-circuited transmission line of FIGS. 9c and 9d, and

FIG. 10b illustrates the result of further fabrication steps applied tothe structure of FIG. 10a;

FIG. 11 illustrates a short-circuited multiple transmission line whichmay be encapsulated as described in conjunction with FIGS. 9c or 9d, andused for interconnecting planar circuit arrangements at frequenciessomewhat lower than the higher RF frequencies, such as the clockfrequencies of logic circuits;

FIG. 12 is a perspective or isometric view of a structure according toan aspect of the invention, including a planar plastic HDI circuit, abipartite separator plate, and a second planar circuit, some of whichare cut away to reveal interior details;

FIG. 13 is an exploded view of the structure of FIG. 12, showing theplanar plastic HDI circuit associated with one portion of the separatorplate as one part, the second portion of the separator plate, and thesecond planar circuit as other parts of the exploded structure;

FIG. 14 is an exploded view of a portion of the second part of theseparator plate, showing rigid and compliant transmission lines, andother structure; and

FIG. 15 is a more detailed cross-sectional view of the structure of FIG.12.

DESCRIPTION OF THE INVENTION

In FIGS. 3a 3b, and 3c, a short transmission line or "molded coaxialinterconnect" 310 is in the form of a flat disk or right circularcylinder 311 having a thickness 312 and an outer diameter 314 centeredabout an axis 308. Thickness 312 should not exceed diameter 314. Anelectrically conductive center conductor 316 is in the form of a rightcircular cylinder defining a central axis which is concentric with axis308. A set 318 of a plurality, in this case eight, of further electricalconductors 318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h, are alsoin the form of right circular cylinders, with axes which lie parallelwith the axis 308 of the flat disk. The further electrical conductorshave their axes equally spaced by an incremental angle of 45° on acircle of diameter 320, also centered on axis 308. The main body ofshort transmission line 310 is made from a dielectric material, whichencapsulates the sides, but not the ends, of center conductor 316 andouter conductors 318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h. Thediameter of circle 320 on which the axes of the outer conductors lie isselected so that the outer conductors lie completely within the outerperiphery of the dielectric disk. A first end of the center conductorand the outer conductors lies adjacent a plane 301, and a second end ofeach lies adjacent to a second plane 302. In a particular embodiment ofthe short transmission line, the thickness 312 is 0.055 in., and thediameter is 0.304 in. In another embodiment, the diameter is the same,but the thickness is 0.115 in. In both embodiments, the axes of theouter conductors of set 308 are centered on a circle of diameter 0.192in., and the conductors have diameters of 0.032 in. The material of thedielectric disk is Plaskon SMT-B-1 molding compound, and the conductorsare copper. As described below, these short transmission lines are usedfor interconnecting RF circuits. The characteristic impedance of theshort transmission line of FIGS. 3a, 3b, and 3c is selected tosubstantially match the impedances of the signal source and sink, or tosubstantially match the impedances of the stripline or microstriptransmission lines to which the short transmission line is connected inan HDI circuit. The impedance Z₀ of the short transmission line isdetermined by ##EQU1## where ε is the dielectric constant of thedielectric disk;

D₀ is the diameter of the inside surface of the outer conductor; and

D_(i) is the outer diameter of the center conductor. To produce a 50-ohmcharacteristic impedance, with center conductor wire diameter of 0.032"and epoxy encapsulation material having a dielectric constant of 3.7,the axes of the outer conductors should be on a circle having a diameterof 0.192 inches.

FIGS. 4a, 4b, 4c, 4d, 4e, 4f, and 4h illustrate steps in the fabricationof an RF HDI structure. In a step preceding that illustrated in FIG. 4a,one or more short transmission lines 310 are fabricated, and monolithicRF circuits 14 are assembled with heat-transferring metal deep-reachshims 18. In FIG. 4a, the chip/shim assemblages 14/18 and the shorttransmission lines 310 are mounted face-down onto an adhesive backedKAPTON substrate 410. FIG. 4b illustrates the encapsulation of theassemblages 14/18 and the short transmission line 310 within an epoxy orother encapsulation to form a structure with encapsulated chips andtransmission-lines. The structure of FIG. 4b with encapsulated chips andtransmission lines then continues through conventional HDI processing.As illustrated in FIG. 4c, vias are laser-drilled to die bond pads 14₁and 14₂ and to the conductors of the short transmission line 310 whichare against the substrate 410. Conductive traces are then patterned onthe exposed substrate 410, making the necessary electrical connections.FIG. 4d illustrates the result of applying a plurality (illustrated asthree) of layers of conductive-trace bearing flexible HDI connectionmaterial designated together as 424, with the traces appropriatelyregistered with the connections 14₁ and 14₂ of the chips 14, and withthe center conductor 316 and the set 318 of outer conductors of theshort transmission line 310.

Following the step illustrated in FIG. 4c, plated through-vias 36 areformed in the conductive-trace bearing flexible HDI connection material424, with the result that the chip connections are made, and theconnections to the short transmission line 18 are made as illustrated inFIG. 4e. The metallization layers 32 connect the short transmission lineto at least one of the chips 14, so that one connection of a chipconnects to center conductor 316 of short transmission line 310 of FIG.4e, and so that a ground conductor associated with the chip connects tothe set 318 of outer conductors of the short transmission line. FIGS. 4frepresents the cutting off of that portion of the encapsulated structure(the structure of FIG. 4e) which lies, in FIG. 4f, above a dash line426. This produces a planar structure 401, illustrated in FIG. 4g, inwhich the connections among the chips 14, and between the chips and oneend of the short transmission lines, lie within the conductive-tracelayers 424 on the "bottom" of the encapsulated structure, and in which aheat interface end 18_(hi) of each of the heat-conducting shims 18, andthe ends of the center conductor 316 and of the set 318 of outerconductors of a coaxial connection structure 490 at the end of the shorttransmission line, are exposed on the "upper" side of the structure ascontacts. The center conductor contact is illustrated as 316_(c), andsome of the outer conductor contacts are designated as 318a_(c) and318f_(c).

FIG. 4h illustrates a cross-section of a structure resulting from afurther step following the step illustrated in conjunction with FIGS. 4fand 4g. More particularly, the structure of FIG. 4g is attached to an RFmanifold or beamformer 430, which distributes the signals which are tobe radiated by the active array antenna. The surface 430s of manifold430 which is adjacent to the encapsulated structure bears conductivetraces, some of which are designated 432. In order to make contactbetween the conductive traces 432 on the RF distribution manifold andthe exposed ends of the center conductor 316 and the set 318 of outerconductors of the short transmission line, compressible electricalconductors 450, termed "fuzz buttons," are placed between the conductivetraces 432 on the distribution manifold 430 and the exposed ends of thecenter conductor 316 and set 318 of outer conductors of each of theshort transmission lines 310. The manifold 430 is then pressed againstthe remainder of the structure, with the fuzz buttons between, whichcompresses the fuzz buttons to make good electrical connection to theadjacent surfaces, and which also tends to hold the fuzz buttons inplace due to compression. Appropriate thermal connection must also bemade between the manifold and the shims 18 to aid in carrying away heat.Thus, in the arrangement of FIGS. 4a-4h, electrical RF signals aredistributed to the ports (only one illustrated) of the distributionmanifold 430 to a plurality of the ports (only one of which isillustrated) represented by short transmission lines 310 of planarcircuit 401 of FIG. 4g, and the signals are coupled through the shorttransmission lines to appropriate ones of the metallization layers 32₀ ,32₁, and 32₂, as may be required to carry the signals to the MMIC foramplification or other processing, and the signals processed by the MMICare then passed through the signal paths defined by the paths defined byconductive traces 32₀, 32₁, and 32₂ to that layer of conductive traceswhich is most remote from the distribution manifold 430. Moreparticularly, when the distribution manifold 430 is in the illustratedposition relative to the encapsulated pieces, the uppermost layer 32₂ ofconductive traces may itself define the antenna elements. Thus, thestructure 400 defined in FIG. 4h, together with other portions whichappear in other ones of FIGS. 4a-4g, comprises the distribution, signalprocessing, and radiating portions of a planar or flat active arrayantenna.

The fuzz buttons 450 of FIG. 4h may be part no. 3300050, manufactured byTECKNIT, whose address is 129 Dermodry Street, Cranford, N.J. 07016,phone (908) 272-5500.

If the conductors 32₂ of metallization layer MT2 of FIG. 4h areelemental antenna elements, the RF manifold 430 can be a feeddistribution arrangement which establishes some measure of control overthe distribution of signals to the active MMICs of the various antennaelements. On the other hand, the structure of FIG. 4h denominated as RFmanifold 430 could instead be an antenna array, with the elementalantennas on side 430p, while the metallization layers 32₁ and 32₂ wouldin that case distribute the signals to be radiated, or collect thereceived signals. Thus, the described structure is simply a connectionarrangement between two separated planar distribution sets.

It will be noted that in FIG. 4h, the region 460 about the fuzz buttons450 is surrounded by air dielectric, which has a dielectric constant ofapproximately 1. Since the fuzz buttons 450 have roughly the samediameter as the center conductor 316 and the outer conductors 318, thecharacteristic impedance of the section 460 of transmission lineextending from exposed traces 432 to short transmission line 310 islarger than that of the short transmission line. If the shorttransmission line has a characteristic impedance of about 50 ohms, thecharacteristic impedance of the region 460 will be greater than 50 ohms.Those skilled in the art know that such a change of impedance has theeffect of interposing an effective inductance into the transmissionpath, and may be undesirable.

FIG. 5 represents a structure such as that of FIG. 4h with a cold plate510 interposed between the HDI-connected chips 10 on structure 12 andthe beamformer 430. The cold plate 510 has an interface surface 510iswhich makes contact with the adjacent surface of the plastic body 12 ofthe HDI circuit 10. The cold plate may be, as known in the art, a metalplate with fluid coolant channels or tubes located within, for carryingheat from heat interface surfaces 18_(hi) to a heat rejection location(not illustrated). Those skilled in the art know that a heat conductivegrease or other material may be required at the interface. Asillustrated in FIG. 5, a fuzz button housing 512 has a thickness aboutequal to that of the cold plate, for holding fuzz buttons 450 in acoaxial pattern similar to that of center conductor 316 and outerconductors 318, for making connections between the center conductor316/outer conductors 318 and the corresponding metallizations 432 of thebeamformer 430. More particularly, the outer conductors 318 and theouter conductor fuzz buttons 450 lie on a circle with diameter d192. Thedielectric constant of the material of fuzz button housing 512 isselected to provide the selected characteristic impedance. As alsoillustrated in FIG. 5, fuzz button housing 512 is not quite as large indiameter as the cut-out or aperture in cold plate 510, in order to taketolerance build-up. Consequently, an air-dielectric gap 512_(g1) existsaround fuzz button housing 512. The axial length of fuzz button housing512 is similarly not quite as great as the thickness of the cold plate510, resulting in a gap 512_(g2). Gaps 512_(g1) and 512_(g2) have aneffect on the characteristic impedance of the transmission path providedby the fuzz buttons 450 which is similar to the effect of the air gap460 of FIG. 4h. In an analysis of an arrangement similar to that of FIG.5, the calculated through loss was 0.8 dB, and the return loss was only10.5 dB.

The fuzz button housing or holder 512 is made from an elastomericmaterial, which compresses when compressed between the HDI-connectedchips 10 and the underlying beamformer 430, so as to eliminate air gapswhich might adversely affect the transmission path. FIGS. 6a, 6b, and 6care views of a compressible or compliant RF interconnect with fuzzbutton conductors. In FIGS. 6a, 6b, and 6c, elements corresponding tothose of FIGS. 3a, 3b, and 3c are designated by like reference numerals,but in the 600 series rather than in the 300 series. As illustrated inFIGS. 6a, 6b, and 6c, compliant RF interconnect 610 includes a fuzzbutton center conductor 616 defining an axis 608, and a set 618including a plurality, illustrated as eight, of fuzz button outerconductors 618a, 618b, 618c, 618d, 618e, 618f, 618g, and 618h, spaced atequal angular increments, which in the case of eight outer conductorelements corresponds to 45°, about center axis 608, on a radius 620having a diameter of 0.200". Dielectric body 611 has an outer periphery611p, and is made from a silicone elastomer having a dielectric constantwithin the range of 2.7 to 2.9, and has an overall diameter 614 of about0.36", and a thickness 612 of 0.10". As can be best seen in FIGS. 6a and6c, the dielectric body 611 has two keying notches 650a and 650b.Dielectric body 611 also has a flanged inner portion 648 with a diameterof 0.30", and the maximum-diameter portion 652 has a thickness 654 ofabout 0.44". The fuzz buttons 616, 618 have a length 613 in the axialdirection which is slightly greater (0.115" in the embodiment) than theaxial dimension 612 of body 611 (0.10"). FIG. 6d illustrates arepresentative one of the outer conductor fuzz buttons, which isselected to be fuzz button 618f for definiteness. In FIG. 6d, outerconductor fuzz button 618f is in the form of a right circular cylindercentered on an axis 617, and defines first and second ends 618f₁ and618f₂ which are coincident with planes 601 and 602, respectively, ofFIG. 6b. The cylindrical form of fuzz button 618f of FIG. 6d defines anouter surface 618_(fs) lying between the first and second ends 618f₁ and618f₂.

FIG. 7 is similar to FIG. 5, and corresponding elements are designatedby the same reference numerals. In FIG. 7, the compliant RF interconnect610 is compressed between the broad surface 430_(fs) of beamformermanifold 430 and the broad surface 712_(ls) of HDI-connected chiparrangement 10, and is somewhat compressed axially, to thereby eliminatethe gap 512_(g2) which appears in FIG. 5. This, in turn, eliminates theprincipal portion of the impedance discontinuity at the interface whichis filled by the compliant RF interconnect 610. The axial compression ofthe dielectric body 611 of the compliant RF interconnect 610, in turn,tends to cause the compliant body 611 to expand radially, to therebysomewhat fill the circumferential or annular gap 512_(g1), which furthertends to reduce impedance discontinuities at the interface. A furtheradvantage of the axial compression of body 611 is that the compressiontends to compress the body 611 around the fuzz button conductors 616,618, to help in holding them in place. Analysis of the arrangement ofFIG. 7 indicated that the through loss would be 0.3 dB and the returnloss 28 dB, which is much better than the values of 0.8 dB and 10.5 dBcalculated for the arrangement of FIG. 5.

As illustrated in FIG. 7, a heat-transfer interface surface 18_(hi) onthe broad surface 712_(ls) of HDI-connected chip structure 10 is pressedagainst cold plate 510.

In the view of FIG. 7, the fuzz button conductors 616 and 618 of thecompliant coaxial interconnect 610 are illustrated as being of adifferent diameter than the conductors 316, 318 of the molded coaxialinterconnect 310, and the outer conductors 618 are centered on a circleof somewhat different diameter than the outer conductors 318. Thedifference in diameter of the wires and the spacing of the outerconductor from the axis of the center conductor is attributable todifferences in the dielectric constant of the epoxy which is used as thedielectric material in the molded coaxial interconnect 310 and thesilicone material which is the dielectric material of compliantinterconnect 610. In order to minimize reflection losses, bothinterconnects are maintained near 50 ohms, which requires slightlydifferent dimensioning. This should not be a problem, so long as thediameters of the circles on which the outer conductors of the molded andcompliant interconnects are centered allow an overlap of the conductivematerial, so that contact is made at the interface.

A method for making electrical connections as described in conjunctionwith FIGS. 6a, 6b, 6c, 7, and 8 includes the step of providing orprocuring a first planar circuit 10 including at least a first broadsurface 712_(ls). The first broad surface 712_(ls) of the first planarcircuit 10 includes at least one region 490 defining a first coaxialconnection. It may also include at least a first thermally conductiveregion 18_(hi) to which heat flows from an active device within thefirst planar circuit. The first coaxial connection 490 of the firstplanar circuit 10 defines a center conductor contact 616_(c) centered ona first axis 608 orthogonal to the first broad surface of the firstplanar circuit 10, and also defines a first plurality of outer conductorcontacts, such as 618a_(c) and 618f_(c). Each of the outer conductorcontacts such as 618a_(c), 618f_(c) of the first coaxial connection 490of the first planar circuit 10 is centered and equally spaced on acircle spaced by a first particular radius, equal to half of diameterdl92, from the first axis 608 of the center conductor contact 616 of thefirst coaxial connection 490. The first broad surface 712_(ls) of thefirst planar circuit 10 further includes dielectric materialelectrically isolating the center conductor contact 616_(c) of the firstplanar circuit 10 from the outer conductor contacts, such as 618a_(c),618f_(c), and the outer conductor contacts, such as 618a_(c), 618f_(c),from each other. The method also includes the step of providing a secondplanar circuit 430, which includes at least a first broad surface430_(fs). The first broad surface 430_(fs) of the second planar circuit430 includes at least one region 431 defining a coaxial connection. Thecoaxial connection 431 of the second planar circuit 430 includes acenter conductor contact 432_(c) centered on a second axis 808orthogonal to the first broad surface 430_(fs) of the second planarcircuit 430, and also includes the first plurality (eight) of outerconductor contacts 432_(o). Each of the outer conductor contacts, suchas 432_(co), 432_(o), of the coaxial connection 431 of the second planarcircuit 430 is centered and equally spaced on a circle spaced by asecond particular radius, close in value to the first particular radius,from second axis 808 of the center conductor contact 432_(c) of thecoaxial connector 431 of the second planar circuit 430. The first broadsurface 430_(fs) of the second planar circuit 430 further includesdielectric material electrically isolating the center conductor contact432_(c) of the second planar circuit 430 from the outer conductorcontacts, such as 432_(co), 432_(o) of the second planar circuit 430,and the outer conductor contacts, such as 432_(co), 432_(o) of thesecond planar circuit 430, from each other. A compliant coaxialconnector 610 is provided, which includes (a) a center conductor 616which is electrically conductive and physically compliant, at least inthe axial direction. The compliant center conductor 616 has the form ofa circular cylinder centered about a third axis 608, and defines anaxial length 613 between first 617_(f1) and second 617_(f2) ends. Thecompliant coaxial connector 610 also includes (b) an outer electricalconductor arrangement 618 including a set 618 including the firstplurality (eight) of mutually identical, electrically conductive,physically compliant outer conductors 618a, 618b, 618c, 618d, 618e,618f, 618g, and 618h. Each of the compliant outer conductors 618a, 618b,618c, 618d, 618e, 618f, 618g, and 618h is in the form of a circularcylinder centered about an axis 617, and each has an axial length 613between first 617_(f1) and second 617_(f2) ends which is equal to theaxial length 613 of the compliant center conductor 616. The axes 617 ofthe compliant outer conductors 618a, 618b, 618c, 618d, 618e, 618f, 618g,and 618h are oriented parallel with each other, and with the third axis608 of the compliant center conductor 616. The first ends 617_(f1) ofthe compliant center conductor 616 and the compliant outer conductors618a, 618b, 618c, 618d, 618e, 618f, 618g, and 618h coincide with a firstplane 601 which is orthogonal to the axes 608, 617 of the compliantcenter conductor 616 and the compliant outer conductors 618a, 618b,618c, 618d, 618e, 618f, 618g, and 618h, and the second ends 617_(f2) ofthe compliant center conductor 616 and the compliant outer conductors618a, 618b, 618c, 618d, 618e, 618f, 618g, and 618h coincide with asecond plane 602 parallel with the first plane 601. The compliant outerconductors 618a, 618b, 618c, 618d, 618e, 618f, 618g, and 618h have theiraxes 617 equally spaced from each other at the particular radius fromthe axis 608 of the compliant center conductor 616. The compliantcoaxial connector 610 further includes (c) a compliant dielectricdisk-like structure 611 defining a fourth center axis 608 coincidentwith the third axis 608 of the compliant center conductor 616 and alsodefining an uncompressed axial length no more than about 10% greaterthan the uncompressed axial length of the compliant center conductor616. The compliant disk-like structure 611 also defines a periphery 611pspaced from the center axis 608 by a second radius which is greater thanboth (a) the first radius (half of diameter 620) and (b) the axiallength 613 of the compliant center conductor 616. The compliantdielectric disk 611 surrounds and supports the compliant centerconductor 616 and the compliant outer conductors 618a, 618b, 618c, 618d,618e, 618f, 618g, and 618h at least on side regions 618_(fs) thereoflying between the first 618_(f1) and second 618_(f2) ends of thecompliant center conductor 616 and the compliant outer conductors 618a,618b, 618c, 618d, 618e, 618f, 618g, and 618h. The compliant dielectricdisk-like structure 611 does not overlie the first 618_(f1) ends of thecompliant center conductor 616 and the compliant outer conductors 618a,618b, 618c, 618d, 618e, 618f, 618g, and 618h, so that electricalconnection thereto can be easily established.

The method described in conjunction with FIGS. 6a, 6b, 6c, 7, and 8 alsoincludes the further step of placing the first broad surfaces 712_(ls),430_(fs) of the first and second planar circuits 10, 430 mutuallyparallel, with the first axis 8 passing through the center of the centerconductor contact 316c of the first planar circuit 10 and orthogonal tothe first broad surface 712_(ls) of the first planar circuit 10, andcoaxial with the second axis 808 passing through the center of thecenter conductor contact 432_(c) of the second planar circuit 430orthogonal to the first broad surface 430_(ls) of the second planarcircuit 430, with the first and second planar circuits 10, 430rotationally oriented around the coaxial first and second axes 8, 808 sothat a fourth axis 880 orthogonal to the first broad side 712_(ls) ofthe first planar circuit 10 and passing through the center of one of theouter conductor contacts 318_(cc) of the first coaxial connector 431 ofthe first planar circuit 10 is coaxial with a fifth axis 882 orthogonalto the first broad side 430_(fs) of the second planar circuit 430 andpassing through the center of one of the outer conductor contacts432_(cc) of the first coaxial connector 431 of the second planar circuit430. The compliant coaxial connector 310 is placed between the first andsecond planar circuits 10, 430, with the third axis 608 of the compliantcenter conductor 616 substantially coaxial with the mutually coaxialfirst and second axes 8, 808. The compliant coaxial connector 610 isoriented so that a sixth axis 884 of one of the compliant outerconductors 618a, 618b, 618c, 618d, 618e, 618f, 618g, and 618h is coaxialwith the mutually coaxial fourth and fifth axes 880, 882. Force isapplied to translate the first and second planar circuits 10, 430 towardeach other until the compliant coaxial connector 610 is compressedbetween the first broad surface 712_(ls) of the first planar circuit 10and the first broad surface 430_(fs) of the second planar circuit 430sufficiently to make contact between the center conductor contacts316_(c), 432_(c) of the first and second planar circuits 10, 430 throughthe compliant center conductor 616, and to make contact between outerconductor contacts 318a_(c), 318f_(c) of the first planar circuit andcorresponding outer conductor contacts 432_(ac), 432f_(c) of the secondplanar circuit 430 through some of the compliant outer conductors 618.

In a particular version of the method described in conjunction withFIGS. 6a, 6b, 6c, 7, and 8 also includes the further step of procuring afirst planar circuit 10 in which the first broad surface 712_(ls)includes a first thermally conductive region 18_(hi) to which heat flowsfrom an active device within the first planar circuit. In this versionof the method, before the step of applying force to translate the firstand second planar circuits 10, 430 toward each other, a planar spacer orcold plate 510 is interposed between the first broad surface 712_(ls) ofthe first planar circuit 10 and the first broad surface 430_(fs) of thesecond planar circuit 430. In this method, the step of interposing aplanar cold plate 510 between the first broad surfaces 712_(ls),430_(fs) comprises the step of interposing a planar cold plate 510having an aperture 810 with internal dimensions no smaller than twicethe second radius of the compliant dielectric disk-like structure 610,with the outer periphery of the aperture 810 surrounding the compliantcoaxial connector 610.

FIG. 9a is a simplified perspective or isometric view of a shortmonolithic (one-piece without joints) conductive short-circuitedtransmission line or RF interconnect 900 according to an aspect of theinvention, FIG. 9b is a side or elevation view of the transmission lineof FIG. 9a, and FIGS. 9c and 9d illustrate the arrangement of FIG. 9a inencapsulated form. In FIGS. 9a and 9b, the short-circuited transmissionline or RF interconnect 900 has an air dielectric, and is made bymachining from a block, or preferably by casting. Transmission line 900includes a center conductor 916 centered on an axis 908, and having acircular cross-section. Center conductor 916 ends at a plane 903 in aflat circular end 916e, and each of the outer conductors 918a, 918b,918c, 918d, 918e, 918f, and 918h also has a corresponding flat circularend 918ae, 918be, 918ce, 918de, 918ee, 918fe, and 918he. Thecross-sectional diameters of the center conductor 916 and the outerconductors 918a, 918b, 918c, 918d, 918e, 918f, and 918h taper from arelatively small diameter d₁ of the circular ends at plane 903 to alarger diameter d₂ at a second plane 902. At (or immediately adjacentto) plane 902, a short-circuiting plate 907 interconnects the ends ofthe center conductor 916 and the outer conductors 918a, 918b, 918c,918d, 918e, 918f, and 918h which are remote from plane 903. In FIGS. 9aand 9b, the axes of outer conductors 918a, 918b, 918c, 918d, 918e, 918f,and 918h, only one of which is illustrated and designated 918aa, lie ona circle illustrated as a dash line 921, which lies at a radius 920 fromaxis 908 of center conductor 916. The periphery lip of short-circuitingplate 907 is illustrated as being circular, with a diameter or radiusmeasured from axis 908 which is just large enough so that the outeredges of the various outer conductors of set 918 are coincident ortangent with periphery llp at plane 902.

While not the best mode of using the short-circuited transmission lineof FIGS. 9a and 9b, FIGS. 9c and 9d illustrate the short-circuitedtransmission line 900 of FIGS. 9a and 9b encapsulated in a cylindricalbody 911 of dielectric material corresponding to the dielectric body 311of FIG. 3, to form an encapsulated short-circuited transmission line901. As illustrated in FIG. 9c, the encapsulating body 911 does notcover the ends 916e and 918ae, 918be, 918ce, 918de, 918ee, 918fe, and918he of the center and outer conductors, thereby making them availablefor connections. As also illustrated in FIG. 9c, the diameter ofdielectric body 911 of encapsulated short-circuited transmission line901 is the same as the diameter 914 of the short-circuiting plate 907,so the side of the short-circuiting plate 907 is exposed. The diameterof the dielectric encapsulating body could be greater than diameter 914of the short-circuiting plate 907, in which case the plate 907 would notbe visible in FIG. 9c.

With the unencapsulated short-circuited transmission-line 900 made asdescribed in conjunction with FIGS. 9a, 9b, or with the encapsulatedshort-circuited transmission line 901 made as described in conjunctionwith FIGS. 9a, 9b, 9c, and 9d, the unencapsulated (900) or encapsulatedtransmission line (901) can then be made a part of a planar circuit. Theunencapsulated short-circuited transmission line 900 of FIGS. 9a and 9b,or the encapsulated transmission line 901, is placed on a substrate 410as illustrated for circuit 310 in FIG. 4a, with its exposed conductorends 916e, 918ae, 918be, 918ce, 918de, 918ee, 918fe, and 918he adjacentsubstrate 410. The steps of FIGS. 4b, 4c, and 4d are followed.

FIG. 10a is a simplified representation of the result of applying thesteps of FIGS. 4a, 4b, 4c, and 4d to the encapsulated transmission line901 of FIGS. 9a, 9b, and 9c. In FIG. 10a, elements corresponding tothose of FIG. 4e are designated by like reference numerals, and elementscorresponding to those of FIGS. 9a, 9b, 9c, and 9d are designated bylike reference numerals. As illustrated in FIG. 10a, the planar circuitstructure 1000, which may be an antenna array, has the location of theshort-circuiting plate 907 below the parting plane 426 at which a cut ismade to expose a newly formed end 1016e of the tapered center conductorand to also expose newly formed ends of the set of outer conductors 918,respectively. As illustrated in FIG. 10a, the parting plane lies betweenplanes 903 and 902 associated with the RF interconnect 900. FIG. 10b isa simplified cross-section of a structure generally similar to that ofFIG. 4h, in which the structure of FIG. 10a is the starting point;elements of FIG. 10b corresponding to those of FIG. 10a are designatedby like reference numerals, and elements corresponding to those of FIG.4h are designated by like reference numerals. It will be apparent tothose skilled in the art that the structure of FIG. 10B is equivalent tothat of FIG. 4h, with the sole difference lying in the tapered diameterof the center conductor 916 and of the outer conductors represented by918b and 918f between the small ends 916e and newly formed large ends1018be and 1018fe, respectively. This taper may change thecharacteristic impedance somewhat between the ends of the RFinterconnect, but this effect is mitigated by the relatively smalltaper, and because the axial length of the RF interconnect is selectedto be relatively short in terms of wavelength at the highest frequencyof operation. Naturally, if one or more unencapsulated short-circuitedtransmission lines 900 are used to make the planar circuit according tothe method described in conjunction with FIGS. 4a, 4b, 4c, 4d, 10a, and10b, the dielectric constant of the encapsulant material of thetransmission line is the same as that of the planar circuit itself. Ifan encapsulated transmission line such as 901 is used to make the planarcircuit of FIG. 10b, it is desirable that the encapsulating materials beidentical.

FIG. 11 illustrates a monolithic electrically conductive structure whichforms multiple short-circuited transmission paths, each consisting of atleast one conductor paired with another; as known to those skilled inthe art, one of the pair may be common with other circuit paths, and maybe used at somewhat lower frequencies than the coaxial structures, downto zero frequency. In FIG. 11, the multiple short-circuited transmissionpaths take the form of a monolithic electrically conductive structure1110, including a baseplate 1112 and a plurality, eleven in number, oftapered pins or posts 1114a, 1114b, 1114c, 1114d, 1114e, 1114f, 1114g,1114h, 1114i, 1114j, and 1114k. The short-circuited multipletransmission-line structure is used instead of the coaxial arrangement900 in the method described in conjunction with FIGS. 4a, 4b, 4c, 4d,10a, and 10b, to make a planar structure. Those skilled in the art knowthat antenna array/beamformer combinations require not only connectionof RF signals, but also require transmission between elements of powerand control signals, which can be handled by the structure made with themultiple transmission paths of FIG. 11.

FIGS. 12, 13, 14, and 15 illustrate a planar plastic HDI circuit 10similar to those described in conjunction with FIGS. 3a, 3b, 3c, 4a, 4b,4c, 4d, 4e, 4f, and 4g. More particularly, planar plastic HDI circuit 10includes a molded interconnect 310 such as that described in conjunctionwith FIGS. 3a, 3b, and 3c, assembled to the substrate 12 as described inconjunction with FIGS. 4a, 4b, 4c, 4d, 4e, 4f, and 4g. The planarplastic HDI circuit 10 is mounted on a stiffening plate 510a, which ispart of a bipartite separation plate 510. First portion 510a of thebipartite separation plate 510 has an aperture 810 formed therein toaccommodate the flanged disk-like body of compliant interconnect 610,with the fuzz-button conductors 616, 618 of the compliant interconnectregistered with the conductors of molded interconnect 310 so as to be incontact therewith.

Second portion 510b of separation plate 510 of FIGS. 12, 13, 14, and 15has a through aperture 1312 including a cylindrical portion, and alsoincluding a recess 1214₂ adjacent side 1310b of second portion 510b ofseparation plate 510, which recess accommodates a hold-down flange 1214.Through aperture 1312 also includes a lip or flange 1314 adjacent side1310c, which aids in holding the body of a rigid coaxial transmissionline 1210 in place. Rigid coaxial transmission line 1210 is similar tomolded interconnect 310, but may be longer, so as to be able to carrysignals through the first and second portions of the separation plate510. Aperture 1312 also defines a key receptacle 1316 which accepts akey 1212 protruding from the body of rigid transmission line 1210. Thenumber of conductors of rigid transmission line 1210 is selected, andthe conductors are oriented about the longitudinal axis of the rigidtransmission line, in such a manner as, when keyed into the aperture1312 in separation plate 510, the conductors each match and make contactwith corresponding conductors of compliant interconnects 610a and 610b.Compliant interconnect 610a is compressed between molded interconnect310 and rigid coaxial transmission line 1210, and is oriented to makethe appropriate connections between the center fuzz button 616 of moldedinterconnect 610a and the center conductor 1210c, and between the outerfuzz buttons 618 of molded interconnect 610a and the outer conductors,one of which is designated 1210o, of the rigid transmission line 1210.

Molded interconnect 610b of FIGS. 12, 13, 14, and 15 is compressedbetween a surface 1210s of rigid transmission line 1210 and face 430s ofsecond circuit 430, and, when the second circuit 430 is registered withseparation plate 510, the center and outer metallizations 1332 and 1334,respectively, of its coaxial port 1331 are registered with thecorresponding center fuzz button 616 and outer fuzz buttons 618 ofcompliant interconnect 610b. The second compliant interconnect 610b isheld in place by flange 1214, which in turn is held down by screws 1216aand 1216b in threaded apertures 1218a and 1218b, respectively.

It will be clear from FIGS. 12, 13, 14, and 15 that when the center axis308 of the center-conductor connection 316c of port 490 of the HDIcircuit 10 are coaxial with the axis 1308 of the center-conductorconnection 1332 of the port 1331 of the beamformer or second circuit430, and with the axes 1408, 1210cca, and 1432ca of the centerconductors of the first compliant interconnect 610a, the rigidtransmission line 1210, and the second compliant interconnect 610b, andthe compliant interconnects are of sufficient length, an electricallycontinuous path will be made between the two center conductor contacts.Similarly, with the center conductors and center conductor contactscoaxial, all that is required to guarantee that the outer conductorsmake corresponding contact is that they have the same number and beequally spaced about the center conductors, and that one of the outerconductors or outer conductor contacts in each piece lie in a commonplane with the common axes of the center conductors. When any one of theeight outer conductors or contacts of any one of the interconnectionelements is aligned with the corresponding others, all of the outerconductors or outer conductor contacts is also aligned with itscorresponding elements.

In the particular embodiment of the invention illustrated in FIGS. 12,13, 14, and 15, the separation plate 510 consists of a stiffener plate510a which is adhesively or otherwise held to the otherwise flexibleplastic HDI circuit 12, and the second portion 510b of separator plate510 is a cold plate, which includes interior chambers (not illustrated)into which chilled water or other coolant may be introduced by pipesillustrated as 1230a and 1230b. In a particular embodiment of theinvention, the planar plastic HDI circuit (only a portion illustrated)defines an antenna array, and the MMIC (not illustrated in FIGS. 12, 13,14, and 15) associated with the planar plastic HDI circuit include chipsoperated as active amplifiers for the antenna elements. The secondcircuit 430 is part of a beamformer which supplies signals to, andreceives signals from, the MMIC associated with the planar plastic HDIcircuit 12.

Other embodiments of the invention will be apparent to those skilled inthe art. For example, while the described flat antenna structure lies ina plane, it may be curved to conform to the outer contour of a vehiclesuch as an airplane, so that the flat antenna structure takes on athree-dimensional curvature. It should be understood that an activeantenna array may, for cost or other reasons, define element locationswhich are not filled by actual antenna elements, such an array is termed"thinned." The term "RF" has been used to indicate frequencies which maymake use of the desirable characteristics of coaxial transmission lines;this term is meant to include all frequencies, ranging from a fewhundred kHz to at least the lower infrared frequencies, about 10¹³ Hz.,or even higher if the physical structures can be made sufficientlyexactly. While the short transmission line illustrated in FIGS. 3a, 3b,and 3c has eight outer conductors, the number may greater or lesser. Thedielectric constant of the dielectric conductor holder of the shorttransmission lines is selected to provide the proper impedance, whereasthe specified ranges are suitable for 50 ohms. While the cold plate hasbeen described as being for carrying away heat generated by chips in thefirst planar circuit 10, it will also carry away heat from thedistribution beamformer. While the diameters of the center and outerconductors have been illustrated as being equal, the center conductormay have a different diameter or taper than the outer conductors, andthe outer conductors may even have different diameters among themselves.

Thus, an aspect of the invention lies in a short electricaltransmission-line (310) which includes a center electrical conductor(316) having the form of a circular cylinder centered about an axis(308). The circular cylinder of the center conductor (316) defines anaxial length (312) between first (plane 301) and second (plane 302) endsof the center conductor (316). An outer electrical conductor arrangement(318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h) comprises aplurality of mutually identical electrical outer conductors (318a, 318b,318c, 318d, 318e, 318f, 318g, and 318h), each being in the form of acircular cylinder centered about an axis, and each having an axiallength between first (plane 301) and second (plane 302) ends which isequal to the axial length of the center conductor (316). The axes of theouter conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h)are oriented parallel with each other and with the axis (308) of thecenter conductor (316). The first ends of the center and outerconductors are coincident with a first plane (301) which is orthogonalto the axes of the center (316) and outer conductors (318a, 318b, 318c,318d, 318e, 318f, 318g, and 318h), and the second ends of the center(316) and outer conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g,and 318h) are coincident with a second plane (302) parallel with thefirst plane. The outer conductors (318a, 318b, 318c, 318d, 318e, 318f,318g, and 318h) have their axes equally spaced from each other at afirst radius (320) from the axis (308) of the center conductor (316).The short electrical transmission-line (310) also includes a rigiddielectric disk (311) defining a center axis and an axial length (312)no greater than the axial length of the center conductor (316). Therigid dielectric disk (311) also defines a periphery spaced from thecenter axis by a second radius (314) which is greater than either (a)the first radius or (b) the axial length of the center conductor (316).The dielectric disk encapsulates, or (311) surrounds and supports thecenter (316) and outer conductors (318a, 318b, 318c, 318d, 318e, 318f,318g, and 318h) on side regions thereof lying between the first andsecond ends of the center (316) and outer conductors (318a, 318b, 318c,318d, 318e, 318f, 318g, for holding the center (316) and outerconductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h) inplace. However, the dielectric disk (311) does not overlie the firstends (the ends coincident with plane 301) of the center (316) and outerconductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h).

In a more particular embodiment, the center conductor (316) defines adiameter (d), and the outer conductors (318a, 318b, 318c, 318d, 318e,318f, 318g, and 318h) each have the same diameter. More particularly,the material of the center (316) and outer conductors (318a, 318b, 318c,318d, 318e, 318f, 318g, and 318h) comprises at least a copper interior,and the material of the dielectric disk (311) is epoxy resin.

A method, according to an aspect of the invention, for producing a flatconnection assembly (400) includes the step of affixing a plurality ofmicrowave integrated-circuit chips (14) to a support (410), withconnections of the chips (14) adjacent to the support (410). A pluralityof short electrical transmission-lines (310) are made or generated. Eachof the short electrical transmission-lines (310) is similar to thatdescribed immediately above. A plurality of the short transmission-lines(310) are applied to the support (410), with the first ends of theconductors adjacent the support (410). The chips (14) and the shorttransmission-lines (310) are encapsulated in rigid dielectric material,to thereby produce encapsulated chips and transmission-lines (FIG. 4b).The support (410) is removed from the encapsulated chips andtransmission-lines (FIG. 4b), to thereby expose a first side (411) ofthe encapsulated chips and transmission-lines (FIG. 4b), and at leastthe connections (14₁, 14₂) of the chips (14) and the first ends(adjacent plane 301) of the center (316) and outer conductors (318a,318b, 318c, 318d, 318e, 318f, 318g, and 318h) of the shorttransmission-lines (310). At least one layer (424) of flexibledielectric sheet carrying a plurality of electrically conductive traces(32₁, 32₂) is applied to the first side of the encapsulated chips andtransmission-lines (FIG. 4b). The flexible dielectric sheet (424)interconnects, by way of some of the traces (32₁, 32₂) and by throughvias (36), at least one of the connections (14₁, 14₂) of at least one ofthe chips (14) with the first end of the center conductor (316) of oneof the transmission-lines (310), and at least one other of theconnections (14₁, 14₂) of the one of the chips (14) to the first ends ofall of the outer conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g,and 318h) of the one of the transmission-lines (310), to thereby producea first-side-connected encapsulated arrangement (FIG. 4e). So muchmaterial is removed from that side (413) of the first-side-connectedencapsulated arrangement (FIG. 4e) which is remote from the first side(411) as will expose second ends (316₂ ; 318₂) of the center (316) andouter conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h) ofthe transmission-lines (310), to thereby produce a first planararrangement (401 of FIG. 4h) having exposed second ends (316₂ ; 318₂) ofthe center (316) and outer conductors (318a, 318b, 318c, 318d, 318e,318f, 318g, and 318h) of the transmission-lines (310). A planarconductor arrangement (430) is applied over the first planar arrangement(401), and adjacent that side (426) of the first planar arrangement(401) which has the exposed second ends (316₂ ; 318₂) of the center(316) and outer conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g,and 318h). The planar conductor arrangement (430) includes a pluralityof individual electrical connections (432) which, when the planarconductor arrangement (430) is registered with the first planararrangement (430), are registered with the ends of the center (316) andouter conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h) ofthe transmission-lines (310). The planar conductor arrangement (430) isregistered with the first planar arrangement (401), and electricalconnections (450) are made between the second ends of the center (316)and outer conductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and318h) of the transmission lines (310) of the first planar arrangement(401) and the individual electrical connections (450) of the planarconductor arrangement (430).

In a particular method according to an aspect of the invention, the stepof making electrical connections comprises the steps of placing acompressible floccule (450) of electrically conductive material betweenthe second ends of each of the center (316) and outer conductors (318a,318b, 318c, 318d, 318e, 318f, 318g, and 318h) of the transmission lines(310) of the first planar arrangement (401) and the registered ones ofthe conductors (432) of the planar conductor arrangement (430), andcompressing the compressible floccule (450) of electrically conductivematerial between the second ends of the center (316) and outerconductors (318a, 318b, 318c, 318d, 318e, 318f, 318g, and 318h) of thetransmission lines (310) of the first planar arrangement (401) and theregistered ones of the connections (432) of the planar conductorarrangement (430), to thereby establish the electrical connections andto aid in holding the compressible floccules (450) in place. The step ofencapsulating the chips (14) and the short transmission-lines (310) indielectric material includes the step of encapsulating the chips (14)and the short transmission-lines (310) in the same dielectric materialas that of the dielectric disk (311).

What is claimed is:
 1. A method for producing a flat antenna array,comprising the steps of:affixing a plurality of microwaveintegrated-circuit chips to a planar support, with connections of saidchips adjacent to said support; generating a plurality of shortelectrical transmission-lines, each of said short electricaltransmission-lines comprising:a center electrical conductor having theform of a circular cylinder centered about an axis, and defining anaxial length between first and second ends; an outer electricalconductor arrangement comprising a plurality of mutually identicalelectrical outer conductors, each being in the form of a circularcylinder centered about an axis, and each having an axial length betweenfirst and second ends which is equal to said axial length of said centerconductor, said axis of said outer conductors being oriented parallelwith each other and with said axis of said center conductor, with saidfirst ends of said center and outer conductors coincident with a firstplane which is orthogonal to said axes of said center and outerconductors, and with said second ends of said center and outerconductors coincident with a second plane parallel with said firstplane, said outer conductors having their axes equally spaced from eachother at a first radius from said axis of said center conductor; and arigid dielectric disk defining a center axis and an axial length nogreater than said axial length of said center conductor, and alsodefining a periphery spaced from said center axis by a second radiuswhich is greater than both (a) said first radius and (b) said axiallength of said center conductor, said dielectric disk surrounding andsupporting said center and outer conductors on side regions thereoflying between said first and second ends of said center and outerconductors, but not overlying said first ends of said center and outerconductors, for holding said center and outer conductors in place;applying said plurality of short transmission-lines to said support withsaid first ends adjacent said support; encapsulating said chips and saidshort transmission-lines in rigid dielectric material, to therebyproduce encapsulated chips and transmission-lines; removing said supportfrom said encapsulated chips and transmission-lines, to thereby expose afirst side of said encapsulated chips and transmission-lines, and atleast said connections of said chips and said first ends of said centerand outer conductors of said short transmission-lines; applying to saidfirst side of said encapsulated chips and transmission-lines at leastone layer of flexible dielectric sheet carrying a plurality ofelectrically conductive traces, and interconnecting, by way of some ofsaid traces and through vias, at least one of said connections of atleast one of said chips with said first end of said center conductor ofone of said transmission-lines, and at least one other of saidconnections of said one of said chips to said first ends of all of saidouter conductors of said one of said transmission-lines, to therebyproduce a first-side-connected encapsulated arrangement; removing atleast so much material from that side of said first-side-connectedencapsulated arrangement remote from said first side as will exposesecond ends of said center and outer conductors of saidtransmission-lines, to thereby produce a first planar arrangement havingexposed second ends of said center and outer conductors of saidtransmission-lines; applying, over said first planar arrangement,adjacent said side of said first planar arrangement with exposed secondends of said center and outer conductors, a planar conductor arrangementincluding a plurality of individual electrical connections which, whensaid planar conductor arrangement is registered with said first planararrangement, are registered with said center and outer conductors ofsaid transmission-lines; registering said planar conductor arrangementwith said first planar arrangement; and making electrical connectionsbetween said second ends of said center and outer conductors of saidtransmission lines of said first planar arrangement and said connectionsof said planar conductor arrangement.
 2. A method according to claim 1,wherein said step of making electrical connections comprises the stepsof:placing, between said second ends of each of said center and outerconductors of said transmission lines of said first planar arrangementand the registered ones of said electrical connections of said planarconductor arrangement, a compressible floccule of electricallyconductive material; and compressing said compressible floccule ofelectrically conductive material between said second ends of said centerand outer conductors of said transmission lines of said first planararrangement and the registered ones of said electrical connections ofsaid planar conductor arrangement, to thereby establish said electricalconnections and to aid in holding said compressible floccules in place.3. A method according to claim 1, wherein said step of encapsulatingsaid chips and said short transmission-lines in dielectric materialincludes the step of encapsulating said chips and said shorttransmission-lines in the same dielectric material as that of saiddielectric disk.